This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Previous work has shown that a small FKBP-derived protein can be engineered to exhibit small molecule ligand-dependent rescue of protein stability. When fused to other proteins of interest (POI) and expressed in cells, these "destabilization domains" (DDs) cause the fusion protein to be rapidly degraded in the absence of the ligand. Addition of the cell-permeable ligand stabilizes the domain and allows the fusion protein to exert its biological effect. Our preliminary work shows DD degradation to be proteasome-dependent, with ubiquitin also implicated in the degradation process. Our hypothesis is that in the absence of ligand, the DDs are specifically targeted by cellular protein quality control machinery for degradation. Because of the variety of successfully targeted POIs fused to the DDs, we further hypothesize that proteins involved in DD recognition and degradation could be general "gatekeepers" of quality control and/or protein folding machinery, members of which have been difficult to identify by other biochemical techniques. By using an efficient DD capture technique and mass spectrometry to identify co-purifying proteins, we hope to identify proteins involved in the recognition and processing of DDs for degradation. Knowledge of general cellular mechanisms for protein quality control may help us devise strategies to combat diseases of protein misfolding, such as Alzheimer's, Parkinson's and prion diseases.